As overseas traveling has become more prevalent in recent years, there is a strong demand for the development of a power supply apparatus which can be used not only within the country but also in foreign countries where the voltage of the commercial power supply is different from that in the country, that is, a power supply which can be applied worldwide for charging appliances such as shavers, electric tooth brushes, and the like.
FIG. 17 is a circuit diagram to show a conventional power supply apparatus described in Patent Document 1. The power supply apparatus shown in FIG. 17 is a self-oscillation type power supply apparatus which operates as described below. First, when a power supply section E0 is connected, power is supplied to a capacitor C20 via a bias resistor R80 so that the capacitor C20 is charged and the gate voltage VG of an FET1 increases. Then, when the voltage VG exceeds a threshold voltage of the FET1, the FET1 turns on and a current ID flows. When the current ID increases and R40×ID exceeds a threshold voltage of a transistor Tr10, the transistor Tr10 turns on discharging the gate capacitance of the FET1. Consequently, the voltage VG decreases, the FET1 starts turning off and a coil current IL1 starts turning off as well. This will result in that the voltage VG rapidly decreases due to the voltage induced in a feedback winding L30 and the FET1 fully turns off.
When the FET1 turns off, the resonance circuit consisting of the capacitor C10 and a primary winding L10 will be in a free oscillation condition and the voltage VG exceeds the threshold voltage of the FET1 again due to the coil current IL1, causing the FET1 to turn on again. In this way, the on and off operations of the FET1 are repeated thereby supplying power to a load E20.
Then, since the FET1 is dominated by the voltage ID·R40 across the resistor R40, that is, by the current ID, excessive current will not flow even in a transient state so that the voltage VG will not be excessively decreased and the oscillation in the resonance circuit will be stabilized.
In addition, an RCC (Ringing Choke Converter) type power supply apparatus is disclosed in Patent Document 2 as a related art of the present invention.
However, since the power supply apparatus in Patent Document 1 is not configured to be worldwide applicable, when it is used in the countries or regions where the voltage of the power supply section E0 is large, the drain-to-source voltage of the FET1 becomes excessively large, which leads to a problem that an FET1 having a large drain-to-source withstand voltage needs to be adopted as the FET1.
On the other hand, since the switching power supply apparatus of Patent Document 2 is an RCC type, its switching element operates in a hard switching mode and, as the result of that, a problem arises in that more noise will be generated and the loss of power will increase. Further, since the switching power supply apparatus of Patent Document 2 is an RCC type and therefore the drain-source voltage of the switching element will not increase to the level of a resonance type even when it is used in the countries or regions where the power supply voltage is large, there is no need of decreasing the drain-source voltage of the switching element. Therefore, the above described problem, which arises in self-oscillation type power supply apparatuses, will not arise.
It is an object of the present invention to provide a power supply circuit and power supply system which can be applied worldwide without using a switching element having a large withstand-voltage and also can supply a stable power to load devices.
Patent Document 1: Japanese Patent Laid-Open No. 08-80042
Patent Document 2: Japanese Patent Laid-Open No. 10-98880.